This application is the national phase under 35 U.S.C. xc2xa7371 of PCT International Application No. PCT/JP00/02438 which has an International filing date of Apr. 14, 2000, which designated the United States of America.
The present invention relates to a thermal protector and, more particularly, to a thermal protector suitable to the use for a secondary battery pack incorporated in a portable computer.
For a secondary battery pack incorporated in a portable computer or the like, a nickel-cadmium battery or a nickel-hydrogen battery has conventionally been used as a secondary battery. In the secondary battery pack using such a secondary battery, a thermal protector of a type such that contacts are opened by utilizing a reversing operation of a bimetal plate is incorporated as a protective means against overheat, overload, short circuit, etc.
The above-described thermal protector has a required internal resistance. The reason for this is that in the event that an excessive current due to overload or short circuit flows, the bimetal plate is reversely operated by self heat generation caused by the internal resistance.
In recent years, the secondary battery has shifted from nickel-cadmium battery or nickel-hydrogen battery to lithium-ion battery. The lithium-ion battery requires exact charge/discharge control. Therefore, the control including protection against short circuit has been accomplished by a control circuit using a semiconductor.
However, in order to further enhance safety, it is desirable to additionally use a safety device of a type. different from the control circuit. For this reason, the battery pack using the lithium battery is also mounted with a thermal protector in many cases.
The thermal protector used for a battery pack incorporating the lithium battery is required to have a low internal resistance in contrast with the conventional thermal protector. Specifically, the thermal protector is required to have a low sensitivity to current, in other words, a current capable of being caused to flow in a state in which the thermal protector is not operated by self heat generation (hereinafter referred to as a non-operating current) is required to be large.
The reason for this is that if the non-operating current is small, when the electrical charge of the lithium battery is released, the thermal protector is operated by self heat generation before the battery is discharged completely, leading to difficulty in the discharge.
On the other hand, the capacity of secondary battery pack tends to increase along with rapid improvement in performance of portable computers in recent years. From this point of view as well, it is desired to increase the non-operating current, that is, to increase a current-carrying capacity.
In order to increase the non-operating current, the internal resistance of the thermal protector has only to be decreased. For this purpose, it can be thought that conductive elements of the thermal protector (a terminal, movable plate, etc.) are made of a low-resistance material. However, there is a limit to a decrease in internal resistance caused by the selection of materials, so that it is necessary to try to make constructional improvement in order to more increase the non-operating current.
The present invention has been made in view of the above situation, and accordingly an object thereof is to decrease the internal resistance to increase the non-operating current.
The present invention provides a thermal protector having a fixed plate provided with a fixed contact in the front end portion and a first terminal for external connection in the rear end portion; a movable plate having elasticity so that a movable contact provided in the front end portion thereof is brought into contact with the fixed contact by the elasticity; a second terminal for external connection connected to the rear end portion of the movable plate; and a bimetal plate, the front end portion thereof being engaged with the movable plate, for driving the movable plate in a direction such that the movable contact is separated from the fixed contact by reversing when a predetermined temperature is exceeded, wherein a part of the movable plate has a double construction provided by folding, and an increase in cross-sectional area due to the double construction substantially decreases the internal resistance.
The double constructed portion is preferably provided in a portion excluding a proximal neck portion of the movable plate.
Also, the present invention provides a thermal protector having a fixed plate provided with a fixed contact in the front end portion and a first terminal for external connection in the rear end portion; a movable plate having elasticity so that a movable contact provided in the front end portion thereof is brought into contact with the fixed contact by the elasticity; a second terminal for external connection connected to the rear end portion of the movable plate; and a bimetal plate, the front end portion thereof being engaged with the movable plate, for driving the movable plate in a direction such that the movable contact is separated from the fixed contact by reversing when a predetermined temperature is exceeded, wherein the rear end portion of the movable plate, the rear end portion of the bimetal plate, and one end portion of the second terminal for external connection are superposed on each other and supported, and the movable plate and the second terminal for external connection are electrically connected to each other in a position close to the movable contact in the superposed support portion to substantially decrease the internal resistance.
Further, the present invention provides a thermal protector having a fixed plate provided with a fixed contact in the front end portion and a first terminal for external connection in the rear end portion; a movable plate having elasticity so that a movable contact provided in the front end portion thereof is brought into contact with the fixed contact by the elasticity; a second terminal for external connection connected to the rear end portion of the movable plate; and a bimetal plate, the front end portion thereof being engaged with the movable plate, for driving the movable plate in a direction such that the movable contact is separated from the fixed contact by reversing when a predetermined temperature is exceeded, wherein the rear end portion of the movable plate and one end portion of the second terminal for external connection are superposed on each other and supported and the rear end portion of the bimetal plate is positioned in front of the superposed support portion, and the movable plate and the second terminal for external connection are electrically connected to each other in a position close to the movable contact in the superposed support portion to substantially decrease the internal resistance.
The movable plate, first terminal for external connection, and second terminal for external connection each are preferably made of a material having an electrical conductivity of 50%IACS and higher. Also, the first and second terminals for external connection each are preferably made of copper.
According to the present invention, the internal resistance is decreased, by which the value of non-operating current can be increased. Therefore, if this thermal protector is applied to a secondary battery pack incorporating a lithium battery, in releasing electrical charge for charging the lithium battery, the electrical charge can be released completely. Also, the resultant increase in current-carrying capacity can accommodate an increase in the capacity of the secondary battery pack.